The evolution of high speed packet access (HSPA) towards higher throughput and lower latencies requires improvements to the physical layer as well as possible changes to the architecture. One improvement that has been proposed is the use of higher-order modulations in the downlink (64-QAM) and the uplink (16-QAM) along with enhanced base station receiver capabilities. Another potential improvement is the use of a shorter transmission time interval (TTI). These improvements would be well-suited to the support of delay-sensitive applications with bursty traffic, such as gaming, or to enhance the quality of non-real-time applications such as TCP transfers.
Such evolution has implications on the optimal way of multiplexing users and allocating resources on the UL. For instance, the use of 16-QAM modulation on the UL implies that the chip-level signal-to-interference ratio (Ec/Io) at the base station is well above 0 dB, rather than being below −10 dB as in typical operation with pre-Release 7 (R7) 3GPP systems. This means that fewer wireless transmit receive units (WTRUs) can simultaneously communicate with a base station.
Another consideration is that for a given average data rate, the percentage of time a WTRU has nothing to transmit due to its buffer being empty will increase with increasing instantaneous data rates. Thus, while using high instantaneous data rates improves the user-plane latency, it also means the burstiness of transmissions increases. The signaling mechanisms currently defined for allocating UL resources are not optimized for such bursty operation.
In the UL, the physical and MAC signaling in support of power control and resource allocation is optimized for a scenario where many WTRUs are transmitting simultaneously and at relatively low bit rates. Such signaling will likely not be suitable to take full advantage of the high-data rate capabilities of evolved HSPA, for the following reasons:
First, the power ratio (or equivalently, data rate) allocation to a given WTRU is persistent in the sense that it remains in effect as long as it is not changed by the Node-B through an absolute or relative grant. Such operation is inefficient in a scenario where the burstiness of transmissions is high as will be the case when higher data rates are introduced. This is because the Node-B would constantly have to modify the allocation of each WTRU to avoid overload while efficiently utilizing the resource.
Secondly, the fast closed-loop UL power control that is required up to R6 to maintain the Quality of Service (QoS) of all WTRUs simultaneously transmitting will not be as important in scenarios where a single, or only a few, WTRU is transmitting at a given time, and is adding unnecessary overhead.
Accordingly, better signaling methods are needed to support high UL data rates in evolved HSPA.